Assert-event takes a ground form, i.e., one with no free variables;
stobjs are allowed but only a single non-stobj value can be
returned. The form is then evaluated and if the result is nil, then a
so-called hard error (see er) results. This evaluation is however not done
if proofs are being skipped, as during include-book (also
see skip-proofs and see ld-skip-proofsp), unless :on-skip-proofs t is
supplied.

This form may be put into a book to be certified (see books), because
assert-event is a macro whose calls expand to calls of value-triple
(see embedded-event-form).